King improves trees with large bird (7)
I believe the answer is:
kestrel
'bird' is the definition.
(kestrel is a kind of bird)
'king improves trees with large' is the wordplay.
'king' becomes 'k' (chess abbreviation).
'improves' indicates anagramming the letters (I've seen 'improving' mean this (letters must be sorted out or rectified)).
'with' says to put letters next to each other.
'large' becomes 'L' (eg in clothes sizes).
'trees' after 'l' is 'ltrees'.
'ltrees' anagrammed gives 'estrel'.
'k'+'estrel'='KESTREL'
(Other definitions for kestrel that I've seen before include "High-flyer" , "Falcon that hovers" , "High flier" , "Small type of falcon" , "Bird of prey" .)